A blowout preventer (BOP) is a large, specialized valve or similar mechanical device, used to seal, control and monitor oil and gas wells to prevent blowouts, the uncontrolled release of crude oil and/or natural gas from a well. They are usually installed in stacks of other valves.
These equipment components must:
- close the top of the hole
- control the release of fluids
- permit pumping into the hole
- allow movement of the inner string of pipe
- be activated independently.
BOP stacks can consist of a combination of the following items:
- Annular BOP (sometimes called a ‘spherical BOP’)
- Rotary annular BOPs, which are used for underbalanced drilling or where drilling is required with the BOP closed. ( This can be necessary if complete primary well control is not possible. They can be used as a diverter.)
- Fixed bore pipe rams, which are designed for the size of each type of drill pipe in the drill string. They are solid and close around the drill pipe.
- Variable bore pipe rams, which adjust to different size pipes as specified.
- Blind or complete shut-off (CSO) rams, which close and seal the hole when the rods are out of the hole.
- Blind shear rams, which are designed to cut the drill pipe in the well and shut-in and completely seal the well in an emergency well control the situation. Shear rams take a lot of force to cut the pipe, especially the tool joints. They are essential items in an underwater BOP stack.
- Drilling spool (sometimes referred to as a ‘mud cross’), which is a connecting component either flanged or hubbed, fitted between BOP equipment, with outlets to the choke manifold and kill line.
- Casing bowl. This is a wellhead component or a profile formed in wellhead equipment in which a casing hanger is located when a casing string has been installed. The casing bowl incorporates features to secure and seal the upper end of the casing string and frequently provides a port to enable access to the annulus.
- Adapter spool or sometimes referred to as a DSA (double studded adapter). This is used to adapt the casing bowl to the components of the stack.
- Spacer spool, which is used to adjust the height of BOP stack components.
- Mud riser/bell nipple/flow nipple. This is a section of pipe with an I.D. equal to or greater than the opening of the BOP connected to the top of the BOP stack, and with a side, an outlet to direct the drilling fluid returns to the shale shaker or pit.
It usually has another inlet for the fill line. The configuration of the BOP stack varies according to the rating and drilling conditions. For example:
- For additional safety when the pipe is out of the well some operators include a blind or CSO ram underneath the top pipe ram BOP or underneath the annular BOP.
- Some operators use a rotary annular BOP above the annular BOP, replacing the mud riser to enable a closed-in well while drilling.
BOP stacks are identified by their pressure rating, internal opening, and arrangement of the BOPs and drilling spool from the bottom of the stack.